The results of recent research in speech perception suggest many parallel problems to explore in non-human primate communication. Among these is the perception by monkeys of their own vocalizations, an area important not only for understanding how non-human primates communicate, but also as a basis for comparison with the processes involved in human speech perception. The existence of parallels in the graded acoustic morphology of vocal signals between humans and many other primates suggests that the search will be a rewarding one. This proposal focuses on employing novel techniques which derive from the development of computer synthesized monkey vocalizations. Synthetic vocalizations provide precise stimuli with all acoustic parameters under experimental control. Such stimuli are a prerequisite in the study of complex auditory perceptual processes, but their use is still limited. The research proposed utilizes synthetic calls in conjunction with recorded vocalizations of Japanese macaques to perform tests of discrimination and classification using operant technique on monkey subjects. It is an outgrowth of other studies on Japanese macaques, the non-human primate species with the best-known vocal repertoire. We propose the following questions for experimental testing on Japanese macaques and control species of monkeys: (a) Are the subtle acoustic variants present in the graded vocal repertoire perceived by the animal? (b) What are the acoustic cues that the macaques use to categorize their vocalizations? (c) Can the phenomenon of categorical perception be demonstrated in this species? This last question is prompted by the finding that human beings perceive some phonemes which are connected by acoustic continua in a discontinuous or categorical fashion. Although originally thought to be a special processing mode, categorical perception is now, however, widely considered to reflect more general properties of mammalian auditory systems. If this is true, we should find evidence of similar perceptual processes in non-human primates.